Nitric oxide formation in dilute hydrogen jet flames: Isolation of the effects of radiation and turbulence-chemistry submodels

Nitric oxide (NO) levels in turbulent flames are sensitive to temperature, O atom concentrations, and residence times at the NO forming conditions. Co nsequently, accurate prediction of NO formation is a challenge for turbulen t combustion models. and NO is a useful scalar to consider when evaluating turbulence-chemistry submodels. Predictions of NO formation in a series of hydrogen flames with varying levels of helium dilution are compared with ex perimental results. Dilution reduces flame radiation to the point where unc ertainties in the radiation calculation have only a small influence on NO e mission. and this allows different models for the coupling of turbulence an d chemistry to be compared in relative isolation from the effects of radiat ion. Calculations using the Probability Density Function (PDF) method and the Co nditional Moment Closure (CMC) method are carried out using the same turbul ence model, radiation model, reduced chemical mechanism, and boundary condi tions, so that similarities and differences between these two approaches fo r the coupling of turbulence and chemistry may be isolated and more fully u nderstood. To accomplish this, flow model constants are adjusted to match m easured axial profiles of mean velocity and mixture fraction in the undilut ed flame, before detailed scalar comparisons are carried out. The optically thin radiation assumption is shown to be appropriate for these hydrogen fl ames, and the effects of flame radiation on NO emission are examined. The s ensitivities of the calculated results to certain boundary conditions and s ubmodels are also quantified. Both turbulence-chemistry models yield good r esults for conditional mean temperature and H2O mole fraction. Results also show that both models yield quantitatively useful predictions for the NO f ormation and emission. With all other submodels being the same, the PDF met hod predicts lower NO levels than does the CMC method. This difference is a ssociated with lower O atom concentrations predicted by the PDF method. Bot h methods underpredict NO formation in the lower portion of the jet flames. This deficiency is attributed to the effects of differential diffusion, wh ich are not included in the models, and to the: influence of heat release o n turbulence structure near the flame base. (C) 1999 by The Combustion Inst itute.